Reducing operating current of an integrated circuit is a key design goal for many products. Reduced operating current permits greater levels of integration and reduces the need for thermal management techniques. In circuit designs for battery operated, portable products, reduced current drain of individual circuits allows longer standby and operating life of the product and increases the time between recharging. Reduced operating current is almost always an important goal of any circuit design.
One way to reduce operating current is using high-value resistors to limit the current. For example, in a 2 volt system, a 1 M.OMEGA. resistor will limit the quiescent current to 2 .mu.A, which is a suitable level for many small signal applications.
However, a typical low-cost CMOS integrated circuit fabrication process does not make available high-value resistors as part of the process flow. Such high-value resistors must be formed from a semiconductor layer having high sheet resistance, which is typically not required by any other component of a CMOS integrated circuit. Provision of high-value resistors can therefore only be achieved by adding such a layer to the process flow, which is prohibitively expensive and usually not an option for the circuit designer, or by using physically large resistors made from a low sheet resistance layer. Since minimizing integrated circuit surface area is a key design goal, use of such large resistors is usually avoided.
A further design goal for voltage reference generators is temperature compensation of the output voltage. A temperature compensated output voltage either tracks another signal's variation with temperature or is temperature invariant. Temperature compensation or temperature coefficient tracking is desirable for increasing the operating range of the circuit. A circuit using a reference voltage which does not properly track another signal will no longer function properly when operating temperature is outside a tracking range.
One particular circuit which requires temperature coefficient tracking is a folded cascode output stage. To match the temperature coefficient of the folded cascode output stage, a voltage reference should have the same temperature characteristic and should match the maximum pull up voltage of the folded cascode stage.
Accordingly, there is a need in the art for a voltage reference generator and associated output circuit which minimizes operating current without the use of high-valued resistors and which provides a temperature compensated output voltage, particularly for use with a folded cascode output stage.